(1) Field of the Invention
This invention relates to a method of releasing an inner mold from a rubbery molding having a plurality of corrugations in its inner longitudinal cavity, and more specifically, to a method of releasing a long integral sheath made of a rubbery elastic insulating material forming a synthetic resin electrical insulator.
(2) Description of the Prior Art
Recently, there has been practically used a synthetic resin electrical insulator disclosed in, for instance, Japanese Patent Application Laid Open No. 73,821/1981 in which a whole surface of a fiber reinforced plastic rod (FRP) excluding metal fitting members attached to the rod at both ends thereof is covered with sheath made of a rubbery elastic insulating material such as silicone rubber, ethylene propylene rubber or the like. Such a synthetic resin electrical insulator has the merits that it is light in weight and has a high mechanical strength. It also can be adapted for various uses by appropriately selecting a material or a shape of the sheath.
In particular, an electrical insulator using a long integral sheath 1 having the corrugation 2 in its inner longitudinal cavity as shown in FIG. 1 has been used recently because its electric insulation performance at the interface between an FRP rod 3 and a sheath 1 is excellent.
Heretofore, when the sheath formed from a rubbery elastic insulating material and having corrugations in the inner longitudinal cavity, as mentioned above, is shaped, a separable outer mold having the corresponding inner surface configuration to the contour of the sheath and an inner mold (mandrel) forming the undulated corrugation in the inner longitudinal cavity of the sheath are employed. A rubbery elastic insulating material such as silicone rubber or ethylene propylene rubber is injected into the space between the outer mold and the mandrel. In the case where the mandrel is released from a molding (shaft) obtained by curing the elastic insulating material between the outer mold and the mandrel, it is conventional to open the outer mold first into two halves, as shown in FIG. 2, and one end of the sheath 1 is fixed onto a support base 4. The mandrel 5 is then pushed out from the molding in the direction of the arrow by means of a releasing device 8.
According to the above releasing method, it is possible to push out the mandrel when the sheath is as relatively short, such as 20-30 cm, but the corrugation portion, more specifically, the projecting ribs of the mandrel are firmly engaged with the molding. Thus, when the mandrel is pushed out from the molding, an excessive force is inevitably applied onto the corrugated portion of the inner surface of the sheath and the corrugated portion is likely to be damaged. Furthermore, the end surface is compressed abnormally and an internal defect is also likely to result. These phenomena are pronounced when a sheath as long as several meters is produced. In summary, it has been almost impossible using conventional methods to release a mandrel from a long molding having a corrugation in its inner longitudinal surface without causing damage to the corrugated portion.